Abstract: A small gas turbine engine including a power turbine shaft and a gearbox input shaft co-axially aligned one with the other and mounted in three bearings. The power turbine shaft is provided with a male conical surface while the gearbox input shaft is hollow and includes a female conical surface adapted to mate with the male conical surface of the power turbine shaft. The power turbine shaft also includes an axial projection being threaded and inserted within the hollow portion of the gearbox input shaft. A nut having a shoulder is threaded onto the threaded projection of the power turbine shaft and engages a shoulder provided within the hollow gearbox input shaft such that torsion applied to the nut will force axial engagement of the conical surfaces. A complementary spline section may be provided, on the one hand, between the nut engaging shoulder and the female conical section on the gearbox input shaft. The complete coupling is within the diameter of the two co-axial shafts.